Conventionally, a communication terminal device (hereinafter, referred to as “a communication terminal”) existing in a communication area of a base station performs processing called intermittent reception to reduce power consumption in a standby time. In the intermittent reception, which is processing in a transmission period in which a paging signal calling the communication terminal is cyclically transmitted from the base station, the communication terminal is in a state where the power is turned on (hereinafter, referred to as “awake state”), and the communication terminal receives the paging signal in the awake state. In the intermittent reception, the communication terminal is in a state where the power is turned off (hereinafter, referred to as “sleep state”) in a period other than the transmission period of the paging signal. That is, the communication terminal intermittently receives the paging signal from the base station (hereinafter, referred to as “handover source base station”) of the area where the communication terminal exists. A period in which the communication terminal intermittently receives the paging signal from the handover source base station is referred to as an intermittent reception period.
In recent years, a plurality of base stations may form a logical group so that location information of the communication terminal that performs the intermittent reception is collectively managed by the plurality of base stations. For example, according to Worldwide Interoperability for Microwave Access (WiMAX), which is a communication standard specified in IEEE802.16e, this group is referred to a Paging Group (PG). The base stations and the communication terminal that belong to the PG share a PG-Identifier (ID), which is group identification information that identifies the PG, and the communication terminal may synchronize the intermittent reception period with the base stations belonging to the same PG. The paging signal received by the communication terminal during the intermittent reception period includes the PG-ID.
Here, although the communication terminal performs handover from the handover source base station to another base station, a handover destination base station may belong to a PG that is different from the PG of the handover source base station. In this case, processing (location update processing) for reporting new location information of the communication terminal to the handover destination base station is performed. The location update processing is performed in a period other than the above-described intermittent reception period. In other words, the location update processing is performed in a period in which the communication terminal is in the sleep state. Therefore, the communication terminal that performs the handover to the base station belonging to the PG that is different from the PG of the handover source base station consumes extra power for the location update processing as compared with the case of the handover to the base station belonging to the same PG.
On the contrary, to reduce if not eliminate the location update processing, there is well-known a technique for cyclically broadcasting the PG-ID of neighbor base stations of the handover source base station from the handover source base station to the communication terminal. According to this technique, the communication terminal receives the PG-ID of each neighbor base station and then determines, based on the received PG-ID, whether the neighbor base station belongs to the same PG as the handover source base station. When performing the handover, the communication terminal preferentially selects, from among the neighbor base stations, the neighbor base station that is determined to belong to the same PG as the handover source base station.
Depending on a moving speed of the communication terminal, the handover may be started even when the communication terminal does not receive the PG-ID of neighbor base stations from the handover source base station. Therefore, there is a technique proposed of the communication terminal for being coupled to the neighbor base station to obtain the PG-ID in advance from the paging signal held by the neighbor base station in a case where the handover is determined to be started even when the communication terminal does not received the PG-ID.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2009-182549